Preheat your oven to 350°F (175°C).
Line a 9x13 inch baking pan with parchment paper.
In a large mixing bowl, whisk together the flour, baking soda, and salt. Set aside.
In another bowl, beat the softened butter and brown sugar together until light and fluffy, about 3-4 minutes.
Beat in the eggs, one at a time, followed by the vanilla extract.
Gradually add the dry mixture to the wet mixture, stirring until just combined.
Gently fold in the oats until evenly distributed.
Melt the dark chocolate in a double boiler or microwave-safe bowl.
Combine the melted chocolate with the sweetened condensed milk and 2 tablespoons of butter. Mix until fully incorporated.
Pour half of the oatmeal mixture into the prepared baking pan. Then pour the chocolate mixture over the oats, and spread the remaining oatmeal mixture on top.
Bake for 25-30 minutes, or until the edges are golden brown and a toothpick comes out with a few moist crumbs.
Allow to cool in the pan for 15 minutes, then lift out using the parchment paper and cool completely on a wire rack.
Slice into squares or bars and enjoy!